Integrated circuits have generally seen an increase in use of on-board memory. Typically, the first on-board memories were in the form of Read Only Memories (ROMs) which did not require any power to obtain the information therein, but the information could not be altered by the user. As the need for more versatility in integrated circuits has increased, the requirement for erasing, or writing over, the stored information has become a necessity. Initially, erasable memory was realized in the form of Erasable Programmable Read Only Memories (EPROMs), which were typically erased by utilizing ultraviolet light, and Electrically Erasable Programmable Read Only Memories (EEPROMs), which are electrically erasable. These memories have the advantage that they require no power to maintain data integrity. However, these types of memories require special programming steps in order to erase and write new information to the memories. As a result, battery back-up systems have been incorporated with low current CMOS circuitry to provide some minimal on-board Read/Write memory.
In battery backup systems, the battery voltage is typically lower than the operating supply voltage. For example, the battery voltage may be approximately three volts, whereas the operating voltage is five volts. In order to prevent current from the power supply flowing into the battery and thus charging the battery, a PN diode is utilized that is reversed biased in the normal operating mode. This charging is undesirable in certain conditions such as where lithium batteries are utilized which can be damaged by charging. This diode is typically realized with a P-well formed in an N-type substrate, with the battery connected to the P-well for sourcing current to the N-type substrate when the operating supply voltage is not connected. The N-type substrate is then operable to be connected to the operating supply voltage when that is higher than the battery voltage. Thus, when the operating supply voltage is connected, the diode is reverse biased.
One problem that occurs in the diode protection system is the presence of a parasitic PNP transistor when the battery is sourcing current. This parasitic PNP transistor is formed by an adjacent P-well device in the N-type substrate that is connected to a voltage lower than the battery voltage, resulting in a reverse biased PN junction. Whenever the adjacent P-well device is connected to a voltage lower than the battery voltage and the external supply is not utilized, current can conduct through the substrate to the adjacent P-well device through this parasitic transistor. This is the result of minority carriers injected into the substrate from the forward biased PN junction of the battery protection diode and collected in the adjacent P-well. This leakage current therefore provides an unexpectedly high drain on the battery, thus reducing its lifetime and the lifetime of the battery backed-up product.